Photo-detectors are used in imaging systems for medical, security and industrial applications. One particular application of photo-detectors is in computed tomography (CT) systems.
In a typical CT system, an X-ray source with a fan-shaped or cone-shaped X-ray beam and a two-dimensional radiation detector array are assembled on a mechanical support structure, known as a gantry. In use, the gantry is rotated around an object to be imaged in order to collect X-ray attenuation data from a constantly changing angle with respect to the object. The plane of the gantry rotation is known as an imaging plane, and it is typically defined to be the x-y plane of the coordinate system in a CT system. In addition, the gantry (or more typically the object) is moved slowly along the z-axis of the system in order to collect X-ray attenuation data for a required length of the object. By way of background, examples of CT systems can be found in U.S. Pat. Nos. 6,144,718 and 6,173,031.
The radiation detectors of current state of the art CT systems typically consist of a two-dimensional array of rare earth metal based scintillators and a corresponding two-dimensional array of silicon photodiode detectors. The present invention is related to the detectors. In order to present 3-D image data useful for the user of the CT system, complex reconstruction algorithms and software are utilised after or during data collection from the photodiode detectors.
A typical prior art detector consists of an array of rows and columns of individual detector elements. Columns are organised in the z-axis direction. The elements in rows are in the imaging plane, and produce sets of data known as ‘slices’. In a medical CT machine, for example, each slice image corresponds to a two-dimensional X-ray image of a thin slice of a human body as seen in the direction of the body axis and the machine z-axis. With current technologies a typical high end detector module (also known as a detector block or detector assembly) consists of 64 rows and 16 or 24 columns of individual detector elements, i.e. 1024 or 1536 elements in total.
In CT imaging systems, the size of the detector in the imaging plane is increased by placing individual detector modules, adjacent to each other to thereby increase the size of the detector in the imaging plane. A complete CT system may typically consist of 35-60 detector modules assembled side-by-side in order to build a complete arc-shaped CT detector.
Typical prior art CT detectors operate in such a way that each photodiode element behaves as a current source whose magnitude is dependent upon the intensity of light impinging upon that particular element. This light signal is integrated outside the photodiode chip by a separate preamplifier. This preamplifier must be permanently connected to the photodiode. There are thus as many preamplifiers in the CT system as there are photodiodes.
A trend in the CT industry is to develop efficient read-out electronics and read-out techniques. There is also a desire to use new technology to lower the cost of less advanced CT systems, thus making better imaging performance available to customers not able to acquire the most expensive systems.
Another trend is to build CT machines with more detector elements in the z-axis direction. A photo-detector with the possibility of expansion in the z-axis direction is known as a ‘tileable’ detector. An increase in the number of detector elements in the z-axis direction has advantages in various imaging applications. However this trend for tileable detectors further exacerbates other problems, as they potentially lead to top-end CT machines with increasing numbers, potentially without limit, of detector elements in the z-axis direction. There is a general desire to limit the problems of growth, cost and power consumption of detectors in high-end CT systems having an increased number of detector elements.
With typical prior art photodetectors, increasing the number of elements in the z direction is challenging since there must be a path for the signal of every single photodiode to the respective preamplifiers and the data acquisition system of the CT machine. A number of solutions have been provided for providing such connections, which are particularly advantageous as the number of detectors in the z direction increase. Examples of such solutions are back-illuminated photodiodes, and other techniques as disclosed in WO2004/012274 and WO1004/017429 for example. However these techniques do not provide a solution for reducing the complexity of the processing electronics since every photodiode element is still required to have a dedicated preamplifier and associated following stages of electronics.
The trend, toward new read-out electronics and read-out techniques for advanced. CT detectors has overlapping goals with the need for lower costs for less-advanced CT detectors.
U.S. Pat. No. 6,396,898 describes an alternative way for arranging the readout of photodiodes in an array which provides a solution to decrease the number of preamplifiers required in the read-out circuitry of a CT detector. An on-chip multiplexing scheme provides for a number of photodiode elements on one photodiode chip to share a common readout line in order to greatly reduce the number of signal outputs from the photodiode chip, and to correspondingly reduce signal interconnections between the photodiode chip and the preamplifier electronics external to it. Each photodiode serves as a charge integrating capacitor from which the integrated charge will be transferred into a charge sensitive preamplifier in a short readout period. A number of photodiode elements may be read out sequentially, in a time multiplexed manner, by connecting the photodiode elements sharing the same readout line one-by-one to the actual readout line and the preamplifier connected to it. In practise this is accomplished by manufacturing a switch for each photodiode element and using these switches to perform the multiplexing function. In the simplest case, such switches can be built using individual MOS transistors.
Whilst this solution simplifies the processing circuitry associated with the photodiode array, it leads to further problems. One problem is that the manufacturing process of the photodiode array requires increased complexity, to include the multiplexing circuitry. Another problem is that due to their proximity to the photodiodes, light from the scintillators or radiation can impair the performance of, or deteriorate, the multiplexer switches.
Embodiments of the present invention aim to address one or more of the above problems.